1. Technical Field
The present invention generally relates to a universal mobile telecommunications system (UTMS), including the UTMS that forms part of the 3rd generation partnership project (3GPP or 3GPP2) based on code division multiple access (CDMA).
The present invention relates to the Release 6 wideband CDMA (also known as “WCDMA”) specification that includes high speed downlink packet access (HSDPA) and a new feature, a so-called “Fractional Dedicated Physical Channel (F-DPCH),” to be used together with HSDPA; and more particularly, relates to a recovery method for a lost signaling connection with HSDPA/Fractional DPCH.
2. Description of Related Problem
In general, FIGS. 1a and 1b show basic diagrams of the UMTS packet network architecture, which is known in the art. In FIG. 1a, the UMTS packet network architecture includes the major architectural elements of user equipment (UE), UMTS Terrestrial Radio Access Network (UTRAN), and core network (CN). The UE (also referred to as a “terminal” herein) is interfaced to the UTRAN over a radio (Uu) interface, while the UTRAN interfaces to the core network (CN) over a (wired) Iu interface. FIG. 1b shows some further details of the architecture, particularly the UTRAN, which includes multiple Radio Network Subsystems (RNSs), each of which contains at least one Radio Network Controller (RNC). In operation, each RNC may be connected to multiple Node Bs which are the UMTS counterparts to GSM base stations. Each Node B may be in radio contact with multiple UEs via the radio interface (Uu) shown in FIG. 1b. A given UE may be in radio contact with multiple Node Bs even if one or more of the Node Bs are connected to different RNCs. For instance, a UE1 in FIG. 1b may be in radio contact with Node B2 of RNS1 and Node B3 of RNS2 where Node B2 and Node B3 are neighboring Node Bs. This may occur, for example, when the UE1 is in a handover situation and there is a change in the connection from one Node B to another. The RNCs of different RNSs may be connected by an Iur interface which allows mobile UEs to stay in contact with both RNCs while traversing from a cell belonging to a Node B of one RNC to a cell belonging to a Node B of another RNC.
The Release 6 WCDMA specifications provide for the use of the “Fractional Dedicated Physical Channel (F-DPCH)” together with HSDPA, which may be implemented in a UMTS such as that shown in FIGS. 1a and 1b. In particular, FIG. 2 shows the proposed Fractional DPCH slot formats in the downlink (DL), and illustrates the fact that there is no space in the DL dedicated physical channel (DPCH) for any data carrying dedicated physical data channel (DPDCH) bits. For example, the current DPCH radio frame includes 15 slots, each slot #i having a DPDCH (Data1), DPCCH (TPC and TFCI), DPDCH (Data2) and DPCCH (Pilot), while the proposed Fractional DPCH slot formats include five different options 1-5 (Option 4 was adopted to the specifications), each having some arrangement of the TPC and Pilot in relation to Tx OFF, where the transmission is off, as shown, but no DPDCH bits.
In effect, the principle of the Fractional DPCH is to have in the downlink direction only a Dedicated Physical Control Channel (DPCCH) and not a Dedicated Physical Data Channel (DPDCH) at all, and thus all traffic in the downlink, including a logical channel carrying the control signaling, Dedicated Control Channel (DCCH) (radio resource control (RRC) signaling etc.), would be carried on HSDPA (e.g., on HS-DSCH).
The problem with this approach is that at the cell edge in the soft handover area the terminal may be receiving several radio links but the HS-DSCH only from a single base transceiving station (BTS) (as HS-DSCH cannot be in soft handover). In the Release 5 specifications, if the measurements indicate that the serving cell with the HS-DSCH has become weak, then the control-signaling-carrying logical channel, DCCH, transmitted on the downlink DPCH (DPCH being formed by DPCCH and DPDCH and thus having a data carrying capability) can be used to transport the control message reconfiguring the HS-DSCH to be coming from another cell. This is because when the UE is in SHO the same DPDCH content is transmitted from all the cells participating in the soft handover, but the HS-DSCH is transmitted only from one cell, and thus losing the signal from one cell does not impact the reception of the channels transmitted on the downlink DPDCH. (See FIG. 1d below.) Now if the DCCH delivering the control signaling is mapped on the HS-DSCH and this cell transmitting the HS-DSCH becomes too weak for reliable signaling (and is lost from an active set of suitable cells in the worst case), then a situation can develop in which there is a DPCH coming from several cells but no possibility to carry signaling in the downlink for the terminal, and thus there is no mechanism for the network to reconfigure the terminal's reception to somewhere else from the HS-DSCH that was lost. The situation can occur even with the Release 5 specifications (e.g. if the DCCH is mapped to the HS-DSCH), although the problem can be avoided simply by mapping the DCCH always on the DPCH (and never on the HS-DSCH). However, with the Fractional DPCH, there are no other options available than to use the HS-DSCH for signaling thus the problem cannot be circumvented.
For example, FIG. 1c shows in particular how the DCCH can be mapped either to HSDPA or DCH channels when in the CELL_DCH state and in parallel showing how DCCH is mapped when in the CELL_FACH state, while FIG. 1d shows that HS-DSCH is always transmitted from one cell only. In operation, if the DCCH is transmitted with DPCH, then in the SHO it is transmitted using all the radio links and thus losing one radio link does not cut the DCCH connection, but when DCCH is transmitted with HS-DSCH then in the SHO if that radio link (RL) is lost then the DCCH connection is lost and can be recovered by moving to CELL-FACH where the DCCH is sent using forward access channel shared by all UEs in the CELL_FACH state.
There is no known prior art that provides a solution to this problem.